1. Field of the Invention
The present invention relates to a rechargeable battery, and more particularly to a rechargeable battery having a current cutoff device that operates to ensure safety when operational malfunctions increase the internal pressure of the battery.
2. Discussion of the Related Art
Representative examples of rechargeable batteries that can be manufactured in a small size with large capacity, include, for example, nickel-hydrogen (Ni—H) batteries, lithium batteries, lithium ion batteries, polymer lithium (PLi) batteries, and the like. These rechargeable batteries may be classified based on their appearance. For example, they may be classified as cylindrical batteries, rectangular batteries, pouch type batteries, and so on.
Also, the secondary batteries may include a jelly-roll electrode element. The jelly-roll electrode is a power generator element manufactured by rolling together a stack of a positive electrodes, a separator, and a negative electrodes. The positive and negative electrodes may be manufactured by coating substrates with corresponding active materials, followed by drying, roll pressing and cutting the coated substrates. The cylindrical battery is manufactured by placing the power generator element into a cylindrical can, injecting electrolyte, and sealing the can. Additionally, the power generator element is compressed into a rectangular shape before placing it in a rectangular can.
In the related art, cylindrical rechargeable batteries include a current cutoff device that cuts off a flow of current with a shaped plate. That is, the shaped plate breaks when the internal pressure of the battery rises above an allowable level due to overcharging or operational failures.
For example, U.S. Pat. No. 5,418,082 discloses a sealed battery that includes a current cutoff device, which is hereby incorporated by reference. A safety valve bends when the internal pressure of the battery rises. A gasket is included near an opening of the battery. The safety valve includes a downward protrusion that is welded to a metallic thin plate. An electrode tab of the power generator element is attached to the safety valve by welding. When the internal pressure of the battery rises and the downward protrusion of the safety valve flexes upward, the safety valve is separated from the metal thin plate, thereby cutting off the flow of current.
In these sealed batteries with current cutoff devices, the flow of current in a sealed battery may or may not be timely cut off. This depends on the state of the welded connections between the protrusion of the safety valve and the thin metal plate. Additionally, welded portions of the safety valve and the thin metallic plate may react with an electrolyte, oxidize, and become more likely to separate from one another via small external impacts after a long period of use. It may have nothing to do with the internal pressure of the battery as there are a number of factors to consider when welding the safety valve and the metal thin plate. This makes it difficult to ensure the operational reliability of the current cut-off device. Therefore, the reliability and safety of the battery is not guaranteed.
To improve the limitations of the above described current cut-off device used in conventional sealed batteries, sealed batteries with a more reliable current cut-off device are disclosed in Korean Laid-Open Patent Application Nos. 2001-81549 and 2001-81550 assigned to the same assignee as the present invention, which are hereby incorporated by reference. A sealed battery disclosed in Korean Laid-Open Patent Application No. 2001-81549 is illustrated in FIG. 1.
As shown in FIG. 1, the sealed battery includes a power generator element 11 that is comprised of a positive electrode, a negative electrode, and a separator that are sealed in a can 10. The sealed battery also includes a flexible plate 13 capable of flexing upward in response to the internal pressure of the battery. The flexible plate is connected to an opening of the can 10 by a gasket 12 and to an electrode tab 11a of the power generator element 11. A current cutoff device 14, a positive temperature coefficient (PTC) element 15, and a terminal cap 16 are sequentially disposed on the top of the flexible plate 13.
The current cutoff device 14 includes an insulating substrate 17 with a traverse 17a in a portion corresponding to a deformable center portion of the flexible plate 13. Additionally, upper conductive thin film 18 and lower conductive thin film 19 are formed on and underneath the insulating substrate 17, respectively; a connector 14a connects the upper conductive thin film 18 and lower conductive thin film 19; and a rupture portion 14b is formed in the traverse 17a of the insulting substrate 17. The upper conductive thin film 18 and the lower conductive thin film 19 are displaced from the center of the insulating substrate 17 in opposite directions, having an orientation where one end of the upper conductive thin film 18 corresponds to the terminal cap 16 and one end of the lower conductive thin film 19 overlaps the upper conductive thin film 18.
In the current cutoff device 14 having the above structure, the flexible plate 13 of the current cutoff device 14 bends upward, as illustrated in FIG. 2. That is, when the internal pressure of the battery rises above an allowable range, the rupture portion 14b formed in the traverse 17a of the insulating substrate 17 ruptures, thereby cutting off the flow of current in the battery. The flow of current can be cut off with improved reliability as the physical deformation of the flexible plate 13 causes the rupture portion 14b to break.
As illustrated in FIG. 2, when the rupture portion 14b ruptures the insulating substrate 17 and is broken into parts, the broken ends of the parts rise toward the terminal cap 16. Accordingly, the insulating substrate 17 is short-circuited from the terminal cap 16 by the exposed upper and lower conductive thin films 18 and 19. As such, the flow of current in the battery cannot be cut off completely with the current cutoff device 14 having the above structure, thereby failing to ensure the safety of the battery. To prevent this problem, the upper and lower conductive thin films 18 and 19 are coated with insulating films. However, the insulating films at the broken ends of the upper and lower conductive thin films 18 and 19 may be ripped off, causing the same problem as above. Furthermore, the insulating films are likely to be damaged when assembling the battery.